1. Field of the Invention
The present invention relates to a power assist device, and more specifically to a power assist device that prevents a workpiece from contacting with a counter workpiece due to vibration of the workpiece, and a method of controlling the power assist device.
2. Description of the Related Art
Generally, on production sites and the like, a robot called a power assist device is used to reduce the burden on workers and to provide enhanced workability. As an example of a technique using such a power assist device, there is one in which, upon turning OFF of a deadman switch, which is used for stopping supply of electric power to an assist-conveyance motor to turn an assist-conveyance state OFF, the rotational speed of the assist-conveyance motor is gradually lowered to zero, thereby suppressing vibration of a workpiece due to an abrupt stop (see, for example, Japanese Patent Application Publication No. 2005-28491 (JP-A-2005-28491)).
The characteristic feature of the assist-conveyance stopping method described in JP-A-2005-28491 is as follows. When the deadman switch is switched OFF, supply of power to a motor that constitutes an assist-conveyance actuator is interrupted. The output shaft of the motor is provided with a brake mechanism that engages when a solenoid is energized, and disengages when the energization is interrupted. With the above configuration, from a time t1 when the deadman switch is turned OFF, a delay td is set longer than the time that it takes for the rotational speed of the motor to become zero, thereby reducing the occurrence of an impact or a device breakdown.
However, the assist-conveyance stopping method disclosed in JP-A-2005-28491 takes no consideration to the risk of contact with another workpiece (counter workpiece) due to vibration of an arm following the running or stopping of a conveyor.
Specifically, if the technique described in JP-A-2005-28491 is applied to, for example, a task of fitting a windshield (it corresponds to a windscreen) to the body of an automobile that is conveyed by a conveyor, the following problems occur upon abrupt stop or restarting (abrupt start) of the conveyor. (1) The windshield and the body come into contact with each other. (2) Adhesive (reference numeral 12 in FIG. 6) applied to the surface of the windshield adheres to the body, and thus needs to be wiped off. Further, because the amount of adhesive decreases due to its adhesion onto the body, if bonding to a predetermined position is performed in this state, the amount of adhesive becomes insufficient, so there is a need to replenish adhesive. (3) When inserting the windshield into a gap between the trunk (it corresponds to a boot) and the body, the windshield hits the trunk or the body.
The above-described problems occur may occur for at least one of the following reasons. (a) The conveyor that conveys the body must be brought to an immediate stop in the event of emergency, resulting in an abrupt stop, so the impact becomes large. (b) Because the entire assist device is advancing in a synchronized manner in the advancing direction of the conveyor that conveys the body, the impact exerted upon the abrupt stop of the conveyor also causes the assist device to vibrate. (c) Because the worker riding on the conveyor is also advancing in synchronization with the advancing of the conveyor that conveys the body, the impact exerted upon the abrupt stop of the conveyor is applied to the operating handle held by the worker. (d) There is a delay between when the worker recognizes the stop of the conveyor, and when the worker actually releases the deadman switch and brake is applied by the actuator. That is, because the plurality of vibrations due to impact described above occur in an overlapping manner, there are such risks as collision between the windshield and the vehicle, adhesion of adhesive to the vehicle, and flatting the adhesive that has adhered.